Process for the carbonylation of conjugated dienes

ABSTRACT

The present invention relates to a process for the carbonylation of conjugated dienes, whereby a conjugated diene is reacted with carbon monoxide and a hydroxyl group containing compound in the presence of a catalyst system including: (a) a source of palladium cations, (b) a phosphorus-containing ligand, (c) a source of anions, wherein the phosphorus-containing ligand is a ligand having the general formula (I): X 1 —R—X 2  wherein X 1  and X 2  represent a substituted or non-substituted cyclic group with at least 5 ring atoms, of which one is a phosphorus atom, and R represents a bivalent organic bridging group, connecting both phosphorus atoms, containing from 1 to 4 atoms in the bridge, whereby the carbonylation process can be performed batch wise, semi-continuously or continuously.

The present invention relates to a process for the carbonylation ofconjugated dienes, whereby a conjugated diene is reacted with carbonmonoxide and a hydroxyl group containing compound in the presence of acatalyst system including a source of palladium cations, aphosphorus-containing ligand and a source of anions. In particular itrelates to the preparation of alkyl pentenoates and/or adipates from1,3-butadiene and derivatives thereof.

U.S. Patent publication No. 5,495,041 describes a process for thepreparation of a pentenoate ester by carbonylation of butadiene in thepresence of carbon monoxide, alcohol and a catalyst system comprisingpalladium, pentenoic acid and a phosphorus-containing ligand. Thephosphorus-containing ligand can be a monodentate or multidentatephosphorus-containing ligand or a mixture thereof. Preferably amonodentate phosphorus-containing ligand is used to obtain a highselectivity. The examples disclose the use of 1,4-bis(diphenyl-phoshino)butane and triphenyl phosphine as phosphorus-containing ligands. Themolar ratio of butadiene to palladium is according to this publication,preferably less than 20:1. A disadvantage is that this catalyst systemhas only a moderate activity.

European patent publication No. 0198521 describes a process forpreparing carboxylic di-esters, or carboxylic di-acids by the reactionof a conjugated diene with carbon monoxide and an alkanol or water inthe presence of an aprotic solvent and a dissolved catalyst systemcomprising a divalent palladium compound, a triaryl phosphine andhydrogen chloride. The catalyst system may further comprise a bidentatephosphorus-containing ligand, i.e. a bis(diaryl-phoshino)alkane. In itsexamples it is illustrated that the process can also be used for thepreparation of mono-esters. The conversion of 1,3-butadiene tocarboxylic diesters or to mono-esters is performed in one step. Because,at least one mole of hydrogen chloride per atom of trivalent phosphoruspresent in the catalytic system is considered necessary, supplementary,cost-increasing measures are required to avoid corrosion. Examplesdisclose that the molar ratio of conjugated diene to palladium is about115:1.

Object of the present invention, is to provide an improved process interms of catalyst activity for carbonylation of conjugated dienes.

An improved process has now been found for the selective conversion ofconjugated dienes such as 1,3-butadiene, with a high conversion rate, inthe presence of a specific catalyst system.

The present invention therefore provides a process for the carbonylationof conjugated dienes, whereby a conjugated diene is reacted with carbonmonoxide and a hydroxyl group containing compound in the presence of acatalyst system based on:

(a) a source of palladium cations,

(b) a phosphorus-containing ligand

(c) a source of anions, wherein the phosphorus-containing ligand is aligand having the general formula I

X¹—R—X²  (I)

wherein X¹ and X² represent a substituted or non-substituted cyclicgroup with at least 5 ring atoms, of which one is a phosphorus atom, andR represents a bivalent organic bridging group, connecting bothphosphorus atoms, containing from 1 to 4 atoms in the bridge.

This specific catalyst system has an unexpectedly high activity, whichallows for molar ratios well over 200:1 and suitably well over 300:1 ofconjugated diene to palladium to be used, whilst still obtaining highselectivities to the desired product(s). High conversion rates areachieved without the necessity of the presence of halides, thus allowingcheaper types of steel for the reactor installations. A furtheradvantage is that in one step simultaneously both mono-esters anddi-esters, and in particular mono-esters and di-esters of butadiene,such as methyl-pentenoate and dimethyl adipate, can be prepared.

A catalyst system comprising palladium cations, a carboxylic acid and1,2-bis(cyclooctylenephosphino)ethane as the phosphorus-containingligand is described in PCT patent publication 9703943 for thecarbonylation of ethene.

However, the process of the present invention is specifically directedto the carbonylation of conjugated dienes, which show specific reactioncharacteristics when compared to olefins in general. Conjugated dienescontain at least two conjugated double bonds in the molecule. Byconjugation is meant that the location of the π-orbital is such that itcan overlap other orbitals in the molecule. Thus, the effects ofcompounds with at least two conjugated double bonds are often differentin several ways from those of compounds with no conjugated bonds. It isgenerally acknowledged that the carbonylation of conjugated dienescomprises more difficulties than that of a mono-olefin. For example, inEuropean patent publication No. 0495548, relating to the carbonylationof olefins with a catalyst system obtainable by combining a group VIIImetal with a bidentate phosphorus-containing ligand, i.e. abis(di(tertiary alkyl)-phoshino)alkane, it is stated that diolefins withmore than one unsaturated double bond may be used, however, inparticular those wherein the double bonds are non-conjugated.

The process of the present invention can advantageously be used toprepare mono-esters and/or diesters in one step. By optimising reactionconditions such as the residence time, the pressure, the temperature,the amount of hydroxyl group containing compound, the source of anionsand specific type of ligand, the process can be made more selective bythe skilled person to either mono-esters or diesters as will bedescribed in more detail below.

Preferred hydroxyl group containing compounds in the process of theinvention are alkanols with 1 to 20, more preferably with 1 to 6 carbonatoms per molecule and alkanediols with 2-20, more preferably 2 to 6carbon atoms per molecule. Suitable alkanols in the process of theinvention include methanol, ethanol, propanol, iso-propanol, butanol,iso-butanol, tert.butanol, pentanol, hexanol, cyclohexanol and phenol.Preferably methanol or ethanol is used as a hydroxyl group containingcompound. Methanol is especially preferred as a hydroxyl groupcontaining compound.

The conjugated diene preferably is a conjugated diene having from 4 to20, more preferably from 4 to 8 carbon atoms per molecule. Theconjugated diene can be substituted or non-substituted. Most preferably,the conjugated diene is non-substituted 1,3-butadiene.

For example when methanol is used as a hydroxyl group containingcompound in the carboxylation process of 1,3-butadiene enables theproduction of methyl-pentenoate and/or dimethyl adipate and preferablyboth. Dimethyl adipate is an intermediate compound in the preparation ofadipic acid, an intermediate compound to prepare Nylon 6,6.Methyl-pentenoate is an important intermediate in the production processof ε-caprolactam. For the preparation of ε-caprolactam,methyl-pentenoate is hydroformylated to methyl formylvalerate, whichafter reductive amination and cyclisation forms ε-caprolactam.ε-Caprolactam is a starting material in the manufacture of Nylon 6fibres or engineering plastics. Hydroformylation, reductive aminationand cyclisation can be performed in any manner known to a person skilledin the art. Suitable processes are given in PCT application No. 95/0625and Japanese patent application No. 25351/66A.

The process of the present invention is therefore particularlyadvantageous when there is a need to supply both methyl-pentenoate, asan intermediate for preparation of Nylon 6, as well as dimethyl adipate,as an intermediate for the preparation of Nylon 6,6. The amount preparedof each intermediate can be varied, in proportion to the commercialneed, by varying the reaction conditions.

The person with average skill in the art will further realise that theprocess of the present invention can also be used to prepare carboxylicmono-acids and/or carboxylic diacids. Carboxylic mono-acids and/orcarboxylic diacids are prepared by reacting conjugated dienes withcarbon monoxide and using water as a hydroxyl group containing compound.In this case, the carbonylation product, i.e. the carboxylic acid ordi-acid can be used as the source of anions.

A suitable source for palladium cations of component (a) are its salts.Preferably, a salt of a carboxylic acid is used, suitably a carboxylicacid with up to 10 carbon atoms, such as salts of acetic acid, propionicacid and butanoic acid, or salts of substituted carboxylic acids such astrichloroacetic acid and trifluoroacetic acid. A very suitable source ispalladium(II) acetate.

The phosphorus-containing ligand of component (b) of the general formulaI

X¹—R—X²  (I)

represents a bidentate ligand wherein X¹ and X² represent a substitutedor non-substituted cyclic group with at least 5 ring atoms, of which oneis a phosphorus atom, and R represents a bivalent organic bridginggroup, connecting both phosphorus atoms, containing from 1 to 4 atoms inthe bridge.

By “a bridge” is understood the shortest connection between bothphosphorus atoms. Preferably, the bridging group R represents analkylene group containing from 1 to 4 carbons atoms in the bridge, butit may also comprise a carbon chain, interrupted by a hetero atom, suchas nitrogen, sulphur, silicon or oxygen atom. The alkylene group can besubstituted or non-substituted or can form part of a aliphatic oraromatic ring structure. Most preferably, R represents a substituted ornon-substituted ethylene or propylene group, connecting both phosphorusatoms by the first and the second or the third carbon atom, such as a1,2-ethylene, a 1,2-propylene or a 1,3-propylene group.

X¹ and X² represent a substituted or non-substituted cyclic group withat least 5 ring atoms, of which one is a phosphorus atom, and preferablywith from 6 to 10 ring atoms. More preferably X¹ and X² represent asubstituted or non-substituted cyclic group with 9 ring atoms, of whichone is a phosphorus atom. By “a cyclic group” is understood a monocyclicor polycyclic group. Preferred cyclic groups are bicyclic groups.Particularly preferred is a substituted or non-substituted bicyclononylgroup wherein one carbon atom is replaced by the phosphorus atom, i.e. a9-phosphabicyclononyl group. The 9-phosphabicyclononyl group can haveseveral isomeric structures. For the purpose of the invention the[3,3,1] and [4,2,1] isomers are preferred. Suitably X¹ and X² aresubstituted or non-substituted [3,3,1] or [4,2,1] 9-phosphabicyclononylgroups. The two 9-phosphabicyclononyl groups can have both the same oreach a different isomeric structure.

One or both of the phosphabicyclononyl rings can be substituted with oneor more suitable hydrocarbyl groups containing carbon atoms and/orhetero atoms. Suitable substituents include groups containinghetero-atoms such as halides, sulphur, phosphor, oxygen and nitrogen.Examples of such groups include chloride, bromide, iodide, thiol, andgroups of the general formula R²—O—, —S—R², —CO—R², —NH₂, —NHR², —NR²R³,—CO—NR²R³, —OH, —PO₄, —NO₂, —NOH, —CO, —SO₂, —SOH, in which R² and R³,independently, represent hydrocarbyl groups like methyl, ethyl, propyland isopropyl. Groups containing carbon-atoms include straight-chain orbranched saturated or non-saturated carbon containing groups. If aphosphabicyclononyl ring is substituted it is preferably substitutedwith a carbon containing group. Such a carbon containing group can,however, contain additional hetero atoms, such as halides, sulphur,oxygen and nitrogen or hetero-groups as described hereinbefore.Preferably, substituted phosphabicyclononyl rings are substituted withalkyl groups, preferably having from 1 to 10 carbon atoms, morepreferably from 1 to 4 carbon atoms. Linear, branched or cyclic alkylgroups can be used. Suitable alkyl groups include, methyl, ethyl,propyl, iso-propyl, butyl and iso-butyl. More suitably methyl groups areused. The substituted phosphabicyclononyl ring can be mono- orpoly-substituted and is preferably di-substituted. Most preferably thephosphabicyclononyl ring is substituted with two methyl groups. Thephosphabicyclononyl ring can be substituted at all carbon atoms of thering. However, the use of rings with substituents on certain carbonatoms can be more beneficial. Suitably, rings are used with substituentson two carbon atoms, suitably carbon atom 1, 2, 8 and carbon atom 4, 5or 6.

Suitable phosphorus-containing ligands of formula (I) thus includesubstituted or non-substituted:

symmetric or asymmetric [3,3,1] or [4,2,1] isomers of 1,3-P,P′bis(9-phosphabicyclononyl) propane;

symmetric or asymmetric [3,3,1] or [4,2,1] isomers of1,2-P,P′bis(9-phosphabicyclononyl)propane;

symmetric or asymmetric [3,3,1] or [4,2,1] isomer of1,2-P,P′bis(9-phosphabicyclononyl)ethane; and mixtures of abovementioned compounds.

The selectivity of the process to either mono-esters or diesters can bepartly regulated by the phosphorus-containing ligand used. Diesters aremost suitably prepared with a catalyst system wherein thephosphorus-containing ligands of formula (I) are non-substituted. Forthe preparation of mono-esters one or both of the phosphabicyclononylrings of the abovementioned phosphorus-containing ligands is preferablysubstituted. Suitable substituted diphosphines for the preparation ofmono-esters include, symmetric or asymmetric [3,3,1] or [4,2,1] isomersof 1,2-P,P′bis(1,5-dimethyl, 9-phospha bicyclononyl)ethane and mixturesthereof or symmetric or asymmetric [3,3,1] or [4,2,1] isomers of1,3-P,P′bis(1,5-dimethyl, 9-phosphabicyclononyl)propane and mixturesthereof or symmetric or asymmetric [3,3,1] or [4,2,1] isomers of1,2-P,P′bis(1,5-dimethyl 9-phosphabicyclononyl)propane and mixturesthereof, or a combination of above mentioned diphosphines.

The above mentioned phosphorus-containing ligands can be prepared bymethods known in the art, such as for example the method known fromBritish patent publication GB-1127965. Suitably thephosphorus-containing ligands can be prepared by refluxing thephosphabicyclononane hydride with 1,2-dibromoethane in acetonitril.After neutralisation with sodium hydroxide the bis(9-phospha-bicyclononyl)ethane can be isolated. The phosphabicyclononanehydride may conveniently be prepared as described by Elsner et al.(Chem. Abstr. 1978, vol. 89, 180154×). Substituted phosphabicyclononanehydride is conveniently prepared by using substituted cyclooctadienes inthe hereinbefore mentioned process described by Elsner et al.

The source of anions of component (c) can essentially be any salt oracid or a combination of salt and acid.

Preferably an acid is used. A wide range of acids can be used, includinghalide acids, such as hydrochloric acid and hydrobromic acid, mineralacids, such as sulphuric acid, nitric acid and phosphoric acid, andorganic acids, such as acetylacetonic acids, sulphonic acids andcarboxylic acids. Preferably, a protonic acid is used, such as sulphuricacid, phosphoric acid and carboxylic acids. More preferably a carboxylicacid is used. When a protonic acid, preferably a carboxylic acid, isused, preferably an acid with a pKa value >1 in aqueous solution at atemperature of 25° C. is used. Especially suitable acids are benzoicacids. Such a benzoic acid can be substituted. Suitable substituentsinclude halide groups, hydrocarbyl groups, carboxy groups and alkoxygroups. Especially preferred benzoic acids include those substitutedwith methyl or methoxy groups, such as 2,6-dimethoxybenzoic acid and2,4,6-trimethylbenzoic acid.

Suitable salts include any salts formed with inert substantiallynon-coordinating cations, such as sodium or potassium. Suitable saltsinclude the salts of the abovementioned acids.

In a preferred embodiment the source of anions is the same as the sourceof palladium cations in using a palladium salt.

In another preferred embodiment, the source of anions is the carboxylicacid, which results from the carbonylation reaction of the conjugateddiene with carbon monoxide and water or obtainable by hydrolysis of theester product, thus the acid derivative of the ester product to beprepared. For example, if the conjugated diene is 1,3-butadiene,component (c) might conveniently be pentenoic acid, pentenoic acid beingthe product of the carbonylation of 1,3-butadiene with carbon monoxideand water. Component (c) of the catalyst system might thus be differentfor each conjugated diene used. The advantage is that any reaction ofthe component (c) with the hydroxyl group containing compound results inthe same product as obtained by the carbonylation of the conjugateddiene with that hydroxyl group containing compound.

The source of anions can furthermore be any combination of hereinbeforementioned acids and salts. The molar ratio of acid and salt can varybetween wide limits and suitably lies in the range of 100:1 to 1:100,more suitably in the range of 100:1 to 1:1.

The selectivity of the process to either mono-esters or diesters can bepartly regulated by the source of anions used. Depending on the otherreaction conditions, the preparation of diesters can, for example, befavoured by the use of an alkoxy substituted benzoic acid or saltthereof, whereas the preparation of mono-esters can for example befavoured by a carboxylic acid as pentenoic acid. In a suitableembodiment for the preparation of diesters, an carboxylic acid iscombined with an under the reaction conditions substantiallyless-coordinating salt.

The selectivity of the process to either mono-esters or diesters canfurther be regulated by means of residence time. Residence time in thereaction vessel can vary with other factors such as the specific ligandused, the source of anions used and the pressure. Optimal residencetimes in the reaction vessel might further be determined dependent ofother factors known to one skilled in the art such as the catalystconcentration used and the size of the reaction vessel. Residence timecan be very short, even in the range of seconds, dependent of acontinuous, semi-continuous or batch-wise process is being used. For abatch-wise process, suitably the residence times are in the range to 48hours, more suitably in the range of 10 minutes to 15 hours. Generally aresidence time near the upper limit of this range favours thepreparation of diesters and a residence time near the lower limit ofthis range favours the preparation of mono-esters.

The selectivity of the process to either mono-esters or diesters canalso be influenced by means of pressure. Reaction pressures may beatmospheric or superatmospheric. In particular the pressures areconveniently ranging from 1 to 100 bar, preferably 1-30 bar.

The use of a low pressure favours the preparation of diesters, while theuse of a higher pressure favours the preparation of mono-esters. Thepressure used for a specific set of reactants and required product maybe determined by simple experimentation. The ratio of mono-esters anddiesters prepared at a certain pressure can further be regulated byother reaction conditions as mentioned herein.

Carbon monoxide partial pressures in the range of 1-65 bar arepreferred. In the process according to the present invention, the carbonmonoxide can be used in its pure form or diluted with an inert gas suchas nitrogen, carbon dioxide or noble gases such as argon.

The molar ratio of conjugated dienes, in particular butadiene, andhydroxyl group containing compound can vary between wide limits andsuitably lies in the range of 1:0.1 to 1:10, more suitably from 1:1 to1:6. Generally a molar ratio near the lower limit of this range favoursthe preparation of diesters and a molar ratio near the higher limit ofthis range favours the preparation of mono-esters. The amount ofhydroxyl group containing compound to be used for the preparation ofmono-esters and/or di-esters needs to be optimised in relation to otherhereinbefore described factors that influence the ratio betweenmono-esters and diesters prepared, such as residence time and pressure.

The selectivity of the process to a certain compound, expressed in apercentage, is defined as: $\frac{a}{b} \times 100\%$

wherein “a” is the amount of conjugated diene that has been convertedinto that certain compound and “b” the total amount of convertedconjugated diene.

It will be clear that a very high selectivity for the conversion of theconjugated diene in the mono-ester or derivatives thereof is obtained atthe cost of the conversion in the diester or derivatives thereof.

The carbonylation process can be performed batchwise, semi-continuouslyor continuously. Preferably a continuous manner of operation is used.The use of a continuous manner of operation conveniently allows for evenhigher molar ratios of conjugated diene to palladium and lower residencetimes to be used, when compared to the use of a batchwise manner ofoperation.

The ratio of moles of phosphorus-containing ligand, i.e. catalystcomponent (b), per mole of palladium cation, i.e. catalyst component(a), ranges from 0.5 to 10, preferably from 1 to 2.

The quantity of, for instance in-situ prepared, carboxylic acid used inthe carbonylation reaction may vary within wide ranges. Preferably, theamount of acid ranges from 1 mole to 1000 mole of acid per mole ofpalladium cation.

The process of the present invention can be used for each range of moleconjugated diene per mole of palladium cation. Conveniently, the amountof catalyst system is small. The ratio of mole conjugated diene per moleof palladium cation can vary between wide limits, suitably in the rangefrom 1×10¹ to 2×⁷ mole conjugated diene per mole of palladium cation,dependent of a continuous, semi-continuous or batch-wise process isbeing used. The use of a continuous manner of operation convenientlyallows for high molar ratios near the upper limit of this range. For abatch-wise process, suitable the ratio of mole conjugated diene per moleof palladium cation is in the range of 2×10² to 2×10⁶ more suitably inthe range of 3×10² to 2×10⁵.

In the process of the invention liquid carbonylation product and/orsurplus of a carbonylation reactant such as conjugated diene or hydroxylgroup containing compound may serve as solvent during the reaction. Itis also possible to perform the reaction in the presence of an extrainert solvent. Suitable extra inert solvents are for example polar,aprotic compounds, such as ketones, ethers, e.g. anisole,tetrahydrofuran or the dimethylether of diethyleneglycol, or sulphones,e.g. sulpholane.

The carbonylation reaction is conveniently carried out at moderatetemperatures, generally ranging from 20 to 200° C., preferably rangingfrom 80 to 180° C.

The invention is also directed to a catalyst system and its use as ahomogeneous catalyst, based on:

(a) a source of palladium cations,

(b) a phosphorus-containing ligand,

(c) a source of anions,

 wherein the phosphorus-containing ligand is a ligand having the generalformula I

X¹—R—X²  (I)

wherein X¹ and X² represent a cyclic group with at least 5 ring atoms,of which one is a phosphorus atom, and R represents a bivalent organicbridging group, connecting both phosphorus atoms, containing from 1 to 4atoms in the bridge and wherein one or both cyclic groups X¹ and X² aresubstituted with one or more alkyl groups having from 1 to 4 carbonatoms.

Preferred embodiments of the catalyst system are the same as thosedescribed hereinbefore. The above catalyst is preferably used as acarbonylation catalyst.

The invention will be illustrated by the following non-limitingexamples.

EXAMPLE 1

A 300 ml magnetically stirred Hastelloy C autoclave (Hastelloy is aTrade Mark) was filled with 40 ml methanol, 40 ml solvent, 0.5 mmolpalladium acetate, 0.6 mmol 1,2-P,P′bis(9-phosphabicyclononyl)ethane(mainly the symmetrical [3,3,1] isomer) and 2 mmol 2,6-dimethoxybenzoicacid. As a solvent anisole was used, which is a non-polar solvent withsolvent properties similar to that of esters. Subsequently the autoclavewas closed, evacuated and hereafter 20 ml (circa 230 mmol) of1,3-butadiene was added, thus obtaining a ratio of about 460:1 moleconjugated diene per mole of palladium cation, and carbon monoxide to aninitial carbon monoxide pressure of 40 bar was added. The autoclave washeated to 170° C. After a residence time of 10 hours, the contents ofthe autoclave were analysed by means of gas-liquid chromatography. Theinitial rate of carbonylation was 240 mol CO/grams atom palladium/hour.The initial rate of carbonylation was defined as the mean rate of carbonmonoxide consumption over the first 30% butadiene conversion. The totalselectivity to butadiene carbonylation products was found to be morethan 95%. The butadiene carbonylation products consisted of 13%pentenoates and 87% diesters. The diesters consisted of 59%dimethyl-adipate, 19% dimethyl 2-methyl glutarate, 9% dimethyl 2-ethylsuccinate glutarate and 13% dimethyl propyl maleate. Traces ofvinylcyclohexene and its carbonylation products were formed.

EXAMPLE 2

Example 1 was repeated, except that instead of 2 mmol2,6-dimethoxybenzoic acid, 5 mmol 3-pentenoic acid was used as a sourceof anions. After a residence time of 10 hours, the contents of theautoclave were analysed. The initial rate of carbonylation was 400 molCO/grams atom palladium/hour. The total selectivity to butadienecarbonylation products was found to be 90%. The butadiene carbonylationproducts consisted of 57% pentenoates and 43% diesters.

Comparative Experiment A

Example 1 was repeated, except that instead of 0.6 mmol1,2-P,P′bis(9-phosphabicyclononyl)ethane, 0.6 mmol1,2-P,P′bis(dicyclohexylphosphine)ethane was used as a phosphoruscontaining ligand. After a residence time of 10 hours, the contents ofthe autoclave were analysed. The initial rate of carbonylation was only60 mol CO/grams atom palladium/hour, only a quarter of the initial rateof example 1. The total selectivity to butadiene carbonylation productswas found to be less than 60%. The butadiene carbonylation productsconsisted of 95% pentenoates and 5% diesters.

EXAMPLE 3

Example 1 was repeated, except that instead of 2 mmol2,6-dimethoxybenzoic acid, 5 mmol 2,6-dimethoxybenzoic acid was used asa source of anions. After a residence time of 5 hours, the contents ofthe autoclave were analysed. The initial rate of carbonylation was 350mol CO/grams atom palladium/hour. The total selectivity to butadienecarbonylation products was found to be more than 95%. The butadienecarbonylation products consisted of 87% pentenoates and 13% diesters.

EXAMPLE 4

Example 1 was repeated, except that the autoclave was filled with 20 mlmethanol, 40 ml anisole, 0.5 mmol palladium acetate, 0.6 mmol1,2-P,P′bis(1,5-dimethyl, 9-phosphabicyclononyl)ethane (mainly thesymmetrical [3,3,1] isomer), 10 mmol 3-pentenoic acid, closed andevacuated. Hereafter 20 ml of 1,3-butadiene and carbon monoxide to aninitial carbon monoxide pressure of 40 bar was added. The autoclave washeated to 150° C. After a residence time of 5 hours, the contents of theautoclave were analysed. The initial rate of carbonylation was 300 molCO/grams atom palladium/hour. The total selectivity to butadienecarbonylation products was found to be more than 95%. The butadienecarbonylation products consisted of 92% pentenoates and 8% diesters.

EXAMPLE 5

Example 1 was repeated except that the autoclave was filled with 20 mlmethanol, 40 ml anisole, 0.5 mmol palladium acetate, 0.6 mmol1,2-P,P′bis(9-phosphabicyclononyl)orthoxylene, 10 mmol2,4,6-trimethylbenzoic acid, closed and evacuated. Hereafter 20 ml of1,3-butadiene and carbon monoxide to an initial carbon monoxide pressureof 40 bar was added. The autoclave was heated to 150° C. After aresidence time of 5 hours, the contents of the autoclave were analysed.The initial rate of carbonylation was 100 mol CO/grams atompalladium/hour. The total selectivity to butadiene carbonylationproducts was found to be 95%. The butadiene carbonylation productsconsisted of 99% pentenoates and 1% diesters.

Comparative Experiment B

This experiment is comparable to the examples given in U.S. PatentSpecification No. 5,495,041. Example 1 was repeated, except that theautoclave was filled with 20 ml methanol, 40 ml anisole, 0.5 mmolpalladium acetate, 1.0 mmol 1,4-P,P′bis(diphenylphosphine)butane, 10mmol 3-pentenoic acid, closed and evacuated. Hereafter 20 ml of1,3-butadiene and carbon monoxide to an initial carbon monoxide pressureof 40 bar was added. The autoclave was heated to 150° C. After aresidence time of 5hours, the contents of the autoclave were analysed.The initial rate of carbonylation was only 70 mol CO/grams atompalladium/hour. The total selectivity to butadiene carbonylationproducts was found to 95%. The butadiene carbonylation productsconsisted of 98% pentenoates and 2% diesters.

Comparative Experiment C

Comparative experiment B was repeated, except that instead of 1.0 mmol1,4-P,P′bis(diphenylphosphine)butane, 0.6 mmol1,4-P,P′bis(diphenylphosphine)butane was used. After a residence time of5 hours, the contents of the autoclave were analysed. The initial rateof carbonylation was only 20 mol CO/grams atom palladium/hour.

Comparative Experiment D

Comparative experiment C was repeated, except that instead of 0.6 mmol1,4-P,P′bis(diphenylphosphine)butane, 0.6 mmol1,2-P,P′bis(diphenylphosphine)ethane was used. The reaction rate wasless than 20 mol CO/grams atom Palladium/hour.

Comparative Experiment E

Comparative experiment D was repeated, except that instead of 0.6 mmol1,2-P,P′bis(diphenylphosphine)ethane, 1.0 mmol1,2-P,P′bis(diphenylphosphine)ethane was used. After a residence time of5 hours, the contents of the autoclave were analysed. The initial rateof carbonylation was 40 mol CO /grams atom palladium/hour. The totalselectivity to butadiene carbonylation products was found to be 60%. Thebutadiene carbonylation products consisted of 80% pentenoates and 20%diesters.

Comparative Experiment F

Comparative experiment E was repeated, except that instead of 1.0 mmol1,2—P,P′bis(diphenylphosphine)ethane, 0.6 mmol1,2-P,P′bis(ditert-butylphosphine)ethane was used. After a residencetime of 5 hours, the contents of the autoclave were analysed. Theinitial rate of carbonylation was only 30 mol CO/grams atompalladium/hour. The total selectivity to butadiene carbonylationproducts was found to be 55%. The butadiene carbonylation productsconsisted of 90% pentenoates and 10% diesters.

In Table I the phosphorus-containing ligands, the sources of anions andthe quantities of methanol, phosphorus-containing ligand and source ofanions are stated for each example. The autoclave temperatures and theresidence times are also stated in Table I. Furthermore the results ofeach example are summarized in Table I. From the results of the examplesit can be concluded that surprisingly the use of the substituted ornon-substituted phosphorus-containing ligands according to the inventionrenders the catalyst system much more active than the conventionallyused phosphorus-containing ligands. As illustrated by comparingcomparative examples A-F with examples 1-5, the initial rate ofcarbonylation can be more than doubled, while still obtaining highselectivities. The use of the substituted phosphorus containing ligandof example 4 is especially advantageous since here a very high initialrate of carbonylation in combination with a very high selectivity tocarbonylation and a very high ratio of mono- to diesters is obtained.

TABLE I Catalyst system (apart from palladium cations) ExamplePhosphorus-containing ligand mmol source of anions mmol 11,2-P,P′bis(9-phosphabicyclononyl)ethane 0.6 2,6-dimethoxybenzoic acid 22 1,2-P,P′bis(9-phosphabicyclononyl)ethane 0.6 3-pentenoic acid 5 A1,2-P,P′bis(dicyclohexylphosphine)ethane 0.6 2,6-dimethoxybenzoic acid 23 1,2-P,P′bis(9-phosphabicyclononyl)ethane 0.6 2,6-dimethoxybenzoic acid5 4 1,2-P,P′bis(1,5-dimethyl, 0.6 3-pentenoic acid 109-phosphabicyclononyl)ethane 5 P,P′bis(9-phosphabicyclononyl)orthoxylene0.6 2,4,6-trimethylbenzoic acid 10 B1,4-P,P′bis(diphenylphosphine)butane 1.0 3-pentenoic acid 10 C1,4-P,P′bis(diphenylphosphine)butane 0.6 3-pentenoic acid 10 D1,2-P,P′bis(diphenylphosphine)ethane 0.6 3-pentenoic acid 10 E1,2-P,P′bis(diphenylphosphine)ethane 1.0 3-pentenoic acid 10 F1,2-P,P′bis(ditert-butylphosphine)ethane 0.6 3-pentenoic acid 10 InitialRate of Carbonylation Selectivity to Ratio Methanol TemperatureResidence Time (mol CO/gram atom carbonylation mono-: Example (ml) (°C.) (hr) Pd/hr) (%) diesters 1 40 170 10  240 >95 13:87 2 40 170 10  40090 57:43 A 40 170 10   60 <60 95:5  3 40 170 5 350 >95 87:13 4 20 150 5300 >95 92:8  5 20 150 5 100 95 99:1  B 20 150 5  70 95 98:2  C 20 150 5 20 — — D 20 150 — <20 — — E 20 150 5  40 60 80:20 F 20 150 5  30 5590:10

What is claimed is:
 1. A process for the carbonylation of conjugateddienes, whereby a conjugated diene is reacted with carbon monoxide and ahydroxyl group containing compound selected from the group consisting ofalkanols and alkane diols in the presence of a catalyst systemincluding: (a) a palladium salt, (b) a phosphorus-containing ligand, (c)a protonic acid having a pKa value of greater than 1, wherein thephosphorus-containing ligand is a ligand having the general formula IX¹—R—X²  (I) wherein X¹ and X² represent a substituted ornon-substituted cyclic group with at least 5 ring atoms, of which one isa phosphorus atom and the rest are carbon atoms, wherein thesubstitution, if any, is provided by one or more hydrocarbyl groupscontaining carbon atoms and/or hetero atoms, and R represents a bivalentorganic bridging group, connecting both phosphorus atoms, containing 2or 3 carbon atoms in the bridge; and wherein the molar ratio ofconjugated diene to palladium cation is at least 300:1.
 2. A process asclaimed in claim 1, wherein the conjugated diene is 1,3-butadiene.
 3. Aprocess as claimed claim 1, wherein the hydroxyl group containingcompound is an alkanol with from 1 to 20 carbon atoms per molecule.
 4. Aprocess as claimed in claim 1, wherein component (b) of the catalystsystem is a phosphorus-containing ligand of formula (I), wherein thebivalent organic bridging group R is an ethylene or a propylene groupconnected via their terminal carbon atoms to the respective phosphorusatoms of X¹ and X².
 5. A process as claimed in claim 1, wherein X¹ andX² represent a substituted or non-substituted bicyclic group with atleast 5 ring atoms, of which one is a phosphorus atom and the rest arecarbon atoms.
 6. A process as claimed in claim 5, wherein X¹ and X²represent a substituted or non-substituted or 9-phosphabicyclononylgroup wherein the substitution, if any, is provided by hydrocarbylgroups containing carbon atoms and/or hetero atoms.
 7. A process asclaimed in claim 6, wherein one or both of the phosphabicylononyl groupsis substituted with one or more alkyl groups having from 1 to 4 carbonatoms.
 8. A process as claimed in claim 1, wherein the protonic acid isselected from the group consisting of sulphuric acid, phosphoric acid,and carboxylic acids.
 9. A process as claimed in claim 1, wherein1,3-butadiene is converted into methyl pentenoate.
 10. A process toprepare ε-caprolactam wherein methyl-pentonoate prepared according tothe process of claim 1 is hydroformylated to methyl formylvalerate whichis subjected to reductive amination and cyclisation to formε-caprolactam.
 11. The process of claim 1 wherein the conjugated dieneis selected from the group consisting of conjugated dienes having from 4to 20 carbon atoms per molecule.
 12. The process of claim 11 wherein theconjugated diene is selected from the group consisting of conjugateddienes having from 4 to 8 carbon atoms per molecule.
 13. The process ofclaim 1 wherein the alkanol has from 1 to 20 carbon atoms per moleculeand the alkane diol has from 2 to 20 carbon atoms per molecule.
 14. Theprocess of claim 13 wherein the alkanol contains from 1 to 6 carbonatoms per molecule and the alkane diol contains from 2 to 6 carbon atomsper molecule.
 15. The process of claim 1 wherein the palladium salt is acarboxylic acid palladium salt.
 16. The process of claim 15 wherein thecarboxylic acid palladium salt contains up to 10 carbon atoms.
 17. Theprocess of claim 8 wherein the protonic acid is a carboxylic acid. 18.The process of claim 17 wherein the carboxylic acid is a benzoic acid.19. The process of claim 18 wherein the benzoic acid is substituted witha substituent selected from the group consisting of halide groups,hydrocarbyl groups, carboxy groups, and alkoxy groups.
 20. The processof claim 19 wherein the substituents are selected from the groupconsisting of methyl and methoxy groups.